Reduction of base-transit time of InP-GaInAs HBTs due to electron injection from an energy ramp and base-composition grading

The dc current gain of InP-GaInAs heterojunction bipolar transistors with varying base thickness and composition was measured. Much larger composition grade values than previously reported were achieved using strain compensation. A simple two-parameter Monte Carlo simulation was developed to interpret the results. The simulation yields an accurate plot of the base transit time versus base thickness. Clear evidence for the reduction of base transit time due to hot electron injection was observed in devices with thin uniform bases. The current gain of 45-nm-thick graded base devices saturated as the grading was increased beyond standard values. Grading did not increase the gain of devices with a 20-nm-thick base.